1. Field of the Invention
The present invention relates to color television receivers and more specifically to comb filters utilized in these receivers to separate the transmitted luminance and chrominance information.
2. Description of the Prior Art
Standard color television systems transmit the luminance and chrominance information separately. Luminance information is line locked to the horizontal scan, while the chrominance information is transmitted on a sub carrier having a frequency of 3.579545 MHz (227.5 X f.sub.h). This choice of sub carrier provide luminance signals Y that are in phase from horizontal scan line to horizontal scan line and chrominance signals C that are 180 degrees out of phase on adjacent horizontal scan lines. At the video level a composite signal is established that is Y+C for one horizontal scan line and Y-C for the next horizontal scan line. These sum and difference video signals are utilized to separate the luminance video from the chrominance video.
In the prior art the composite video signal is coupled to a comb filter wherein it is delayed one horizontal line time, the delayed signal then being added to and subtracted from the composite signal of the next horizontal line; the sum providing a luminance video Y, the difference providing the chrominance video C. The combed chrominance signal is coupled through a low pass filter and summed with the combed luminance to restore some of the lost luminance vertical detail components, and also coupled through a bandpass filter for further processing.
Filters utilizing one horizontal scan line delay (1-H comb filter) effectively separate the chrominance and luminance signals on vertically correlated video signals thereby eliminating cross color and dot crawl. When the signals are not vertically correlated, however, the 1-H comb filter does not completely separate the Y and C components. This causes "hanging dots" on strong vertical color transitions and cross color components on high frequency diagonal Y transitions, especially when the diagonal detail is angled close to 45 degrees. The "hanging dots" artifact of 1-H comb filters is particularly offensive, since the luminance bandwidth in receivers employing 1-H comb filters is increased approximately 1 MHz over the 3 MHz utilized in older TV receivers wherein a 3.58 MHz trap is used. If the overall amplitude response of the receiver is of the Murakami type for optimum transient response and the phase response is linear, the amplitude of signals at frequencies at 3.58 MHz vicinity are considerably peaked, thus aggravating the "hanging dots" problem by making the dots very visible.